The replication of the retrovirus human T-cell leukemia virus type 1 (HTLV-1) is linked to the development of lymphoid malignancies and inflammatory diseases. Data from in vitro, ex vivo, and in vivo studies have revealed that no specific treatment can prevent or block HTLV-1 replication and therefore that there is no therapy for the prevention and/or treatment of HTLV-1-associated diseases in infected individuals. HTLV-1 and human immunodeficiency virus type 1 (HIV-1) integrases, the enzymes that specifically catalyze the integration of these retroviruses in host cell DNA, share important structural properties, suggesting that compounds that inhibit HIV-1 integration could also inhibit HTLV-1 integration. We developed quantitative assays to test, in vitro and ex vivo, the efficiencies of styrylquinolines and diketo acids, the two main classes of HIV-1 integrase inhibitors. The compounds were tested in vitro in an HTLV-1 strand-transfer reaction and ex vivo by infection of fresh peripheral blood lymphocytes with lethally irradiated HTLV-1-positive cells. In vitro, four styrylquinoline compounds and two diketo acid compounds significantly inhibited HTLV-1 integration in a dose-dependent manner. All compounds active in vitro decreased cell proliferation ex vivo, although at low concentrations; they also dramatically decreased both normalized proviral loads and the number of integration events during experimental ex vivo primary infection. Accordingly, diketo acids and styrylquinolines are the first drugs that produce a specific negative effect on HTLV-1 replication in vitro and ex vivo, suggesting their potential efficiency for the prevention and treatment of HTLV-1-associated diseases.Human T-cell leukemia virus type 1 (HTLV-1) and human immunodeficiency virus type 1 (HIV-1) are exogenous retroviruses pathogenic for humans. Although both viruses are lymphotropic, their pathogenicities depend on strongly distinct mechanisms. Schematically, in vivo, HIV infection triggers the progressive elimination of CD4 ϩ lymphocytes, leading to immunosuppression, whereas HTLV-1 infection is associated with the clonal expansion of infected cells, possibly leading to malignant CD4 ϩ proliferation or to spinal cord infiltration, infection, and inflammation. Clinically, HIV-induced cellular defects are regularly linked to the development of AIDS, whereas in a minority of carriers, HTLV-1 infection causes adult T-cell leukemia/lymphoma (ATLL) and/or tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). The median length of survival for patients with AIDS receiving modern treatment, i.e., triple therapy, is currently over 8 years; in contrast, the prognosis for HTLV-1-associated diseases remains extremely poor. To date, there is no effective treatment for TSP/HAM (32), while the median overall length of survival for patients with ATLL does not exceed a few months (3).Integration of a DNA copy of the viral RNA genome into host cellular DNA is essential and unique to the retroviral life cycle. After completion of reverse...